The restrictions are spelled out in a series of directives — called appropriations riders — attached to the ATF's funding, which tell them what the center can't spend money on, said Charles Houser, chief of the Tracing Center.

"The goal, as I see it, for Congress is to ensure that we don't construct — through some process here — an artificial registration system that begins to infringe on the right to bear arms," Houser said.

These restrictions began in 2003, created by former U.S. Rep. Todd Tiahrt, a Republican from Kansas.

John Spencer of the ATFMPR Photo/Brandt Williams

According to a report by the non-partisan Congressional Research Service, the first Tiahrt amendments were passed at a time when ATF trace data were being used by gun control advocates and some municipalities to file lawsuits against gun manufacturers and gun dealers identified as frequent suppliers of guns used in crimes.

Tiahrt's amendments forbid trace data from being used in civil suits against gun dealers. They also restricted the release of ATF trace data solely to law enforcement agencies involved in "bona-fide criminal investigations." The amendments were supported by the ATF and the Fraternal Order of Police, but opposed by gun control advocates, local police officers and some former ATF agents.

"Tiahrt is in simple terms, nuts," said Joe Vince, a retired ATF agent who worked for the bureau for nearly three decades. Throughout his career, Vince has overseen several different branches of the agency, including its crime gun analysis department.

Vince said the Tiahrt amendments stopped the flow of regional trace data ATF used to share with police departments, regardless of whether they requested the information for an investigation. Without trace data from neighboring cities, he said, police departments lost a resource to help them nab interstate gun traffickers or rogue gun dealers supplying weapons to criminals in their jurisdictions.

"If you arrest a criminal with a firearm, you've taken that firearm off that criminal. If you arrest a trafficker, on average in a year, you're taking 56 guns off the streets that would have went to criminals," Vince said. "If you arrest a source where the firearms are coming from, you're taking well over 600 firearms from hitting the street and being used by criminals."

Vince is a partner in a consulting firm called Crime Gun Solutions based in Frederick, Md. He said the firm's representatives teach police officers how to track down gun traffickers using available trace data. They also testify on behalf of law enforcement agencies and city governments that are suing gun dealers.

BEFORE THE AMENDMENTS

Before the Tiahrt amendments, the ATF provided local law enforcement agencies with trace data that helped officers solve a variety of violent gun crimes, said Gerald Nunziato, chief of the National Tracing Center from 1991 to 1999.

In those days, the ATF offered local departments key information "by having the computer analyze the data and giving the police officer leads," said Nunziato, who retired from the ATF and is Vince's business partner.

The bureau provided local police with addresses, the heights and weights of people linked to guns and information on people who bought a large volume of handguns in a short period of time.

"That's not available to them," he said.

Over the years, Congress has clarified the language in the Tiahrt amendments. Some say the amendments never actually prohibited the ATF from sharing trace data with law enforcement.

In 2008, Congress modified the amendments to state that Tiahrt does not prohibit the release of general statistical data on gun trafficking. A change implanted last year allows the ATF to share firearm trace data among law enforcement agencies that aren't involved in an investigation. However, the ATF is still prohibited from sharing the data with law enforcement if the data would blow the cover of an undercover officer or identify an informant.

A POLITICAL FIGHT

Regardless of the changes, opponents of the Tiahrt amendments, including the International Association of Chiefs of Police and Mayors Against Illegal Guns, still want them abolished.

But gun rights groups and other supporters say restrictions on the ATF are necessary to help preserve the Constitutional rights of gun owners. Besides restricting access to ATF trace data, the Tiahrt amendments require the Justice Department to destroy records of buyers who pass FBI background checks within 24 hours. Tiahrt also prohibits the ATF from requiring gun dealers to conduct annual inventory checks.

U.S. Rep. John Kline, R-Lakeville, said the Tiahrt amendments help prevent the federal government from overreaching its authority.

"There is I think legitimate fear about turning more and more information about gun owners over to the federal government. And so, I resist that," Kline said. "This isn't just a Republican issue or a Democrat issue. There is very strong bipartisan opposition to the federal government, to the Bureau of Alcohol, Tobacco and Firearms and Explosives for taking this step."

A bill with bi-partisan support called the ATF Modernization Act is currently working through Congress. The act would change how the ATF assesses penalties to licensed gun dealers who violate the law by requiring fines to be based upon the nature and severity of the violation. It also would allow a firearms dealer to sell off his inventory for 60 days after he loses his license and require the ATF to prove that a dealer "willfully" mishandled gun records before sanctioning and shutting down the dealer.

U.S. Rep. Keith Ellison, D-Minneapolis, opposes the bill. He said it would let negligent gun makers and sellers off the hook.

"Quite frankly, the bills are designed to try to protect gun manufacturers from any level of accountability for the products they sell," Ellison said. "And this is truly too bad. It's the kind of thing that anybody who doesn't want to get shot needs to stand up against in a bold and aggressive way."

Gun rights lobbying groups such as the NRA and Gun Owners of America support measures such as the Tiahrt Amendment and the ATF Modernization Act.

Gun Owners of America's offices are located in Springfield, Virginia, just outside Washington D.C.

The Modernization Act doesn't go far enough to curtail the power of the ATF, said Larry Pratt, director of Gun Owners of America. Pratt, who said ATF agents regularly hassle gun dealers over minor violations, thinks that the ATF shouldn't be allowed to keep any data on people who buy guns.

"Look at Wikileaks. The government leaks like a sieve," Pratt said. "There's no way they have anyway of seriously protecting the information on gun owners that they're obtaining and keeping illegally. We are just amazed that the Congress hasn't gone farther in restricting what this agency does."

Pratt's group wants to abolish the ATF. But that probably won't happen anytime soon, he said.

As the debate over gun restrictions continues, so does the flow of firearms into Minneapolis. Yet police point out that their strategies of searching for guns obtained illegally, and trying to get them out of the hands of felons, are paying off. They point out that overall, violent crime is down in Minneapolis.